We investigate the one-loop supersymmetric QCD (SUSY-QCD) and electroweak (SUSY-EW) corrections to the top quark decay into a b quark and a longitudinal or transverse W boson. The corrections are presented in terms of the longitudinal ratio Gamma(t-->W(L)b)/Gamma(t-->Wb) and the transverse ratio Gamma(t-->W(-)b)/Gamma(t-->Wb). In most of the parameter space, both SUSY-QCD and SUSY-EW corrections to these ratios are found to be less than 1% in magnitude and they tend to have opposite signs. The corrections to the total width Gamma(t-->Wb) are also presented for comparison with the existing results in the literature. We find that our SUSY-EW corrections to the total width differ significantly from previous studies: the previous studies give a large correction of more than 10% in magnitude for a large part of the parameter space while our results reach only few percent at most.